Foldable bicycles are known to the person skilled in the art of bicycles. Such a bicycle is composed, in particular, of a frame that is made up of a rear portion supporting a rear wheel, and of a front portion supporting a front wheel. In addition, a hinge system is implemented between the rear portion and the front portion so that said front portion can be folded back against said rear portion when the bicycle is in a folded mode. Thus, the bicycle can be disposed in a folded position making the bicycle more compact or in an unfolded position making it possible to use the bicycle. Such foldable bicycles are known, for example, from Documents EP 2 077 222 and EP 0 996 565. In those two documents EP 2 077 222 and EP 0 996 565, the hinge system that is used is provided with locking means enabling the hinge system to be locked in position once the bicycle is unfolded, i.e. once the front portion of the frame extends in forward alignment with the rear portion of said frame, in the in-use position. In Document EP 2 077 222, the locking means consist of a screw-fastening system that locks the front portion in position relative to the rear portion. In Patent Document EP 0 996 565, the locking means consist of a pin that prevents a saddle tube from pivoting, thereby preventing the hinge system from being actuated. A drawback with the hinge systems described in those documents EP 2 077 222 and EP 0 996 565 is that their design makes it difficult for the bicycle to go from the folded position to the unfolded position, and vice versa. Such hinge systems are, in particular, designed to enable the bicycle to be folded for relatively long periods of time. Another drawback suffered by the hinge systems described in those documents EP 2 077 222 and EP 0 996 565 is that they lock the front portion and the rear portion of the bicycle frame relative to each other only in the unfolded position, in order to enable the bicycle to be used.